Martin Schwartz, PhD

Overview and Philosophy

Regulation of cell behavior by adhesion to extracellular matrix (ECM) and mechanical forces are fundamental facts of multicellular life. Cell adhesion to ECM critically regulates cell survival, growth, gene expression and function. Integrins are the major membrane receptors that mediate adhesion of cells to ECM; they also connect the actin cytoskeleton inside the cell to the ECM to provide mechanical integrity, and transmit signals that depend on the composition, organization and mechanical properties of the matrix. Similarly, mechanical forces are fundamental to life, serving as critical guides for morphogenesis and repair. Mechanical forces are especially critical in the cardiovascular system, whose primary function is pumping and delivery of blood to the tissues. Mechanical forces from blood flow, both fluid shear stress, the frictional force from blood flow, and wall stress from blood pressure, are critical for the development, maintenance, physiology and major diseases of the vascular system.

My lab has built an integrated, multi-disciplinary program that combines biophysical, cellular and whole animal approaches to study these problems. We aim to unravel fundamental mechanisms of mechanotransduction through integrins and shear stress receptors, to elucidate the cellular signaling networks that mediate effects on cell behavior and gene expression, and animal models to understand how these events play out during development, in normal physiology, and in vascular diseases such as atherosclerosis.

My laboratory has therefore developed an integrated program addressing interesting problems in cell adhesion, signaling and mechanotransduction. We are currently working in 4 major areas.

Mechanotransduction by integrins

Cells sense the biochemical and mechanical properties of their ECM and respond accordingly [1,3,4].They also respond to external forces applied through the ECM [2]. Exhaustive evidence has shown that integrins mediate these responses but the molecular mechanisms are not well understood. We are currently investigating the role of integrin conformation and the cytoskeletal adapter talin in these processes, and relevance to aneurysms.

These studies have been greatly facilitated by our development of a fluorescence-based method to measure forces across specific proteins in live cells [1]. Studies using this approach with the focal adhesion protein talin, which connects integrins to the actin cytoskeleton, showed that this protein bears force and that it plays a role in how cells sense matrix stiffness [2,5]

Fluid shear stress mechanotransduction in the vascular system

Flowing blood exerts a frictional force called fluid shear stress on the endothelial cells that line the vessels; this force is a major determinant of vascular development, physiology and disease. Atherosclerosis arises in regions of arteries subject to disturbances in fluid flow patterns, while physiological levels of laminar fluid shear stress suppresses inflammatory and atherosclerotic pathways. We have identified complex between VE-cadherin, PECAM-1 and VEGFR2 as a critical mechanotransducer that mediates a subset of these effects [5]. Continuing studies on the junctional complex elucidated mechanisms of force transmission [7] and additional mechanisms [8]. Our continuing studies in mice are investigating how these molecular processes determine physiological vascular remodeling and development of atherosclerosis.

One major pathway downstream of the junctional complex involves activation of integrins, binding to the subendothelial extracellular matrix and subsequent signaling.An important consequence of this pathway is that cell responses to flow are modulated by the ECM. We have found that basement membrane proteins promote flow-dependent activation of anti-inflammatory pathways; by contrast, endothelial cells on provisional ECM proteins such as fibronectin activate multiple inflammatory pathways [4,9]. Further studies identified a mechanism by which fibronectin promotes inflammatory activation of the endothelium by binding and activating the cAMP-specific phosphodiesterase 4D5. This results in decreased signaling through the anti-inflammatory cAMP/protein kinase a pathway, thus creating a cellular state that is permissive for activation by inflammatory mediators. We are now using small molecule approaches to investigate the potential for PDE4D as a therapeutic target in vascular disease.

Collateral artery formation

Blockage of a coronary artery after myocardial infarction leads to downstream ischemia and myocardial cell death. A major mechanism of resistance and recovery is that blockage of an artery triggers increased flow through parallel vessels, which then remodel to accommodate the higher flow. In human patients, this ability to form collateral arteries that perfuse the affected region is a major determinant of recovery after MI. However, the key steps by which high flow stimulates arterialization of small vessels are not well understood, nor are the reasons why some patients are unable to do so. We have elucidated key regulatory circuits by which low and high flow (that is, below and above physiological levels) induce inward vs outward remodeling and applied these methods to improving vascular health in mouse models of disease [10,11].

Aneurysms

Healthy arteries remodel to resist the applied forces forces from blood pressure, to become thicker or thinner when blood pressure increases or decreases, respectively. Aneurysms represent a form of pathological remodeling where the artery wall expands thins and weakens under too-high pressure, eventually resulting in rupture or separation of the layers of the vessel wall, events that are often fatal. Aneurysms may be caused by very high blood pressure in normal people or normal blood pressure in people that carry mutations in genes that mediate mechanosensing or mechanical strength. We have elucidated both homeostatic mechanisms by which cells of the vessel wall sense mechanical properties to maintain normal properties [12] and mechanisms by which abnormal remodeling leads to inflammatory activation to accelerate disease [3].

References

1. Grashoff, C., et al., Measuring mechanical tension across vinculin reveals regulation of focal adhesion dynamics. Nature, 2010. 466(7303): p. 263-6.

2. Kumar A. et al, Filamin A mediates isotropic distribution of applied force across the actin network. J Cell Biol. 2019 218(8):2481-2491.

3. Chen M, et al. FN (Fibronectin)-Integrin α5 Signaling Promotes Thoracic Aortic Aneurysm in a Mouse Model of Marfan Syndrome. Arterioscler Thromb Vasc Biol. 2023;43(5):e132-e150.

4. Yun S. et al, Integrin α5β1 regulates PP2A complex assembly through PDE4D to control atherosclerosis. J Clin Invest. 2019, 130:4863-4874.

5. Driscoll TP, et al. Actin flow-dependent and -independent force transmission through integrins. Proc Natl Acad Sci U S A. 2020 Dec 22;117(51):32413-32422.

6. Tzima, E., et al., Identification of a mechanosensory complex that mediates the endothelial cell response to fluid shear stress. Nature, 2005. 437: p. 426-431.

7. Conway, D.E., et al., Fluid shear stress on endothelial cells modulates mechanical tension across VE-cadherin and PECAM-1. Curr Biol, 2013. 23(11): p. 1024-30.

8. Tanaka K, et al. Latrophilin-2 mediates fluid shear stress mechanotransduction at endothelial junctions. EMBO J. 2024 Aug;43(15):3175-3191.

9. Yun, S., et al., Interaction between integrin alpha5 and PDE4D regulates endothelial inflammatory signalling. Nat Cell Biol, 2016. 18(10): p. 1043-53.

10. Deng H. et al, Activation of Smad2/3 signaling by low fluid shear stress mediates artery inward remodeling..Proc Natl Acad Sci U S A. 2021 Sep 14;118(37):e2105339118.

11. Deng H, et al. A KLF2-BMPER-Smad1/5 checkpoint regulates high fluid shear stress-mediated artery remodeling. Nature Cardiovascular Research 2024, 3: 785-798.

12. Chanduri MVL, et al. Mechanosensing through talin 1 contributes to tissue mechanical homeostasis. Science Advances 2024 PMID: 38328095,